Add memcpy(), memmove() and memset() to the standard library.

Issue #183.

tests/memset-uniform.ispc

@@ -0,0 +1,16 @@
+
+export uniform int width() { return programCount; }
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    int32 * uniform buf = uniform new int32[1024];
+    buf[0] = 0;
+    memset(buf+1, 0x7f, 1024*sizeof(uniform int32));
+
+    int v = buf[programIndex];
+    RET[programIndex] = (v == 0x7f7f7f7f);
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 1;
+    RET[0] = 0;
+}